System and method for operating a high temperature fuel cell as a back-up power supply with reduced performance decay

ABSTRACT

A method is provided for reducing degradation in a fuel cell assembly, including at least one fuel cell with a PBI membrane, during standby, operation. The method may include electrochemically consuming an oxidant from a cathode coupled to the PBI membrane in response to a disconnection of an external load and supplying fuel to remove or electrochemically consume any back-diffused oxidant to the associated fuel cell sufficient to replace or consume the back-diffused oxidant while the external load is removed, and/or also may include controlling a standby temperature of the fuel cell. In this way, it may be possible to avoid increased cell voltage decay associated with degradation of the PBI in a simple and cost effective system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/986,271, filed Nov. 19, 2007 and titled SYSTEM AND METHOD FOROPERATING A HIGH TEMPERATURE FUEL CELL AS A BACK-UP POWER SUPPLY WITHREDUCED PERFORMANCE DECAY, the entire contents of which are incorporatedherein by reference for all purposes.

BACKGROUND

Systems that integrate fuel cells with improved high temperatureperformance can offer advantages as back-up power applications. Forexample, fuel cell systems using a polybenzimidazole (PBI) membrane canoperate at higher levels of carbon monoxide, without auxiliary systemsfor product water management, reactant gas humidification, and simplerheat management.

In back-up power applications, immediate power delivery from the fuelcell system can be facilitated in PBI membrane based fuel cell systemsby maintaining the fuel cells in a standby mode near the nominaloperating temperature range for these types of fuel cells. However,extended durations in standby mode can dramatically reduce performanceand life of the fuel cells.

SUMMARY

The inventors have recognized that performance decay of the fuel cellscan result from deteriorations in the PBI membrane under some conditionspresent during a non-operating standby mode. In one embodiment, a methodis provided for reducing degradation in a fuel cell assembly, includingat least one fuel cell, during such mode. The method may includecreation of an inert environment in the fuel cell by consuming residualoxidant from a cathode coupled to the PBI membrane entering into standbymode in response to a disconnection of an external load, and supplyingfuel to the anode electrode to electrochemically consume anyback-diffused oxidant to the fuel cell while the external load isremoved. The degradation is further reduced if the fuel cell temperatureis reduced in this state to below the normal operating temperature fornominal power generation. In this way, it may be possible to avoid orreduce conditions during standby mode that can lead to increased cellvoltage and corresponding degradation with a simple and cost effectivesystem.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of a back-up powersupply system including a fuel cell system and various auxiliarycomponents of the fuel cell system;

FIG. 2 shows an embodiment of a fuel cell including a polybenzimidazole(PBI) membrane at various stages of degradation;

FIG. 3 shows a high level flow chart depicted an embodiment of a methodfor operating a fuel cell including a PBI membrane to reduce performancedecay; and

FIGS. 4-5 are graphs depicting data illustrating the effects ofdegradations in a PBI membrane on the performance of the fuel cell.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT(S)

FIG. 1 shows a schematic illustration of a fuel cell system 100. Fuelcell system 100 includes a fuel cell assembly 102. In general, the fuelcell assembly 102 may be configured to generate power from a reactionbetween a supplied fuel and oxidant for driving an external load. Thefuel cell assembly 102 may include at least one fuel cell 103. In someembodiments, the fuel cell assembly 102 may include a plurality of fuelcells that may be electrically connected to generate a higher voltage.For example, the fuel cell assembly 102 may include a fuel cell stackincluding a plurality of fuel cells electrically connected in series. Itmay be appreciated that the embodiment illustrated in FIG. 1 shows thecomponents of one fuel cell although the fuel cell assembly includes aplurality of fuel cells connected to fuel cell 103 in series at thedashed lines.

The fuel cell 103 includes an electrolyte 104 disposed between a cathodeelectrode 106 and an anode electrode 108. The cathode and anodeelectrodes may include gas diffusion layers configured to distributeoxidants to the cathode electrode 106 and fuel to the anode electrode108. When fuel is supplied to the fuel cell 103, a voltage is generatedacross the cell by reactions at the cathode electrode 106 and the anodeelectrode 108. As such, the fuel cell 103 may include a cathode flowfield plate 110 configured to direct an oxidant supplied via a cathodeinlet 114 to the cathode electrode 106. Similarly, an anode flow fieldplate 112 may be configured to direct a fuel supplied via an anode inlet116 to the anode electrode 108. Further, a separator plate 118 may bedisposed between the anode and cathode flow field plates between twoadjacent fuel cells in a fuel cell stack. The separator plate 118 may beadapted to direct a flow of heat exchange fluid to control a temperatureof the fuel cell assembly through an internal flow path within theseparator plate 118. In operation of the fuel cell, the separator plate118 may also facilitate electron transport between the anode flow field112 of one fuel cell and the cathode flow field 110 in another fuel cellin a fuel cell stack.

Continuing, the electrolyte 104 transports ions between the cathodeelectrode 106 and anode electrode 108, As such, the electrolyte 104 mayinclude various suitable material or materials, depending upon thechemistry of a specific fuel cell. Suitable materials for an electrolyteinclude materials that exhibit high ion conductivity, low gaspermeability, high mechanical stability, high chemical stability, andhigh thermal stability.

In some embodiments, the fuel cell is a proton exchange membrane (PEM)fuel cell. In such fuel cells, the electrolyte 104 may include aproton-conducting material configured to transport protons generated atthe anode. Some PEM fuel cells may utilize an electrolyte 104 that isoperable at higher temperatures. Such an electrolyte 104 may include apolymer adapted to increase anhydrous ion transfer at highertemperatures while reducing gas permeability. Specifically, a solidpolymer with improved permeability properties doped with a liquidelectrolyte may be operable at higher temperatures with reduced gaspermeability. In one example, a polybenzimidazole (PBI) membrane may bedoped with a strong oxo-acid, such as phosphoric acid and/or sulphuricacid, to increase proton conductivity. The PBI membrane doped with aconcentration of aqueous phosphoric acid (as referred to hereinafter as“the PBI membrane”) may be operable approximately between 160 and 180degrees Celsius.

Fuel is oxidized at the anode electrode 108, thereby producing electronsand protons. For example, hydrogen gas supplied to the anode electrode108 may be ionized, producing electrons (e⁻) and protons (H⁺) accordingto the following reaction:

2H₂→4H++4e−  a.

The hydrogen ions (H+) travel through the electrolyte 104 to the cathodeelectrode 106. Electrons generated at the anode electrode 108 travelthrough the external circuit 117 and pass through the separator plate118 and the cathode flow field 110 to the cathode 106. Supplied oxidantmay react with the electrons (e−) and the hydrogen ions (H+) at thecathode electrode 106 according to the following reaction:

O₂+4e⁻+4H⁺→2H₂O

A reactant delivery system 138 may regulate the supply of the fuel andthe oxidant to the fuel cell assembly 102. The reactant delivery system138 may include a reactant delivery controller 140, an oxidant valve 142for regulating a supply of an oxidant to the fuel cell assembly 102, anda fuel valve 144 for regulating a supply of a fuel to the fuel cellassembly 102. In some embodiments, the oxidant may include oxygen fromcylinder and/or compressed air. As such, the reactant delivery system138 may include an air pump (not shown) to supply air through theoxidant valve to the cathode manifold. It will be understood that thedepicted reactant delivery system 138 is shown for the purpose ofexample, and that any other suitable component or components may beutilized to supply the reactants to the fuel cell assembly 102.

The reactant delivery controller 140 may prompt the oxidant valve 142 toselectively open to deliver an amount of oxidant to the cathodeelectrode 106 of the fuel cell assembly 102. Similarly, the reactantdelivery controller 140 may prompt the fuel valve 144 to deliver anamount of fuel to the anode electrode 108 of the fuel cell assembly 102.

The fuel cell system 100 may include a heat control system 146 toregulate the temperature of the fuel cell assembly 102. The heat controlsystem 146 may include a heat controller 147, a heat exchanger 148including a heat exchange element 150, a fan 152, and a pump 154 forcontrolling the temperature of a heat transfer fluid through a coolingloop 156. The cooling loop 156 may direct the cooling fluid through theseparator plate 118, and/or through any other suitable portions of thefuel cell assembly 102 and/or the fuel cell system 100 to facilitatetemperature control. The heat controller 147 may operate the fan 152 andthe pump 154 to vary a level of heat exchange between the heat exchangeelement 150 and the cooling fluid at the heat exchanger 148. Forexample, the heat controller 147 may selectively activate the fan 152 tocontrol the temperature of the cooling fluid and adjust the flow of thecooling fluid through the cooling loop 156 so as to control thetemperature of the fuel cell assembly 102. As such, the heat controlsystem 146 may further include a fuel cell temperature sensor 158 and acooling loop sensor 160 configured to determine the temperature of thefuel cell assembly and the cooling fluid through the cooling loop 156.

Power generated by the fuel cell assembly 102 may be supplied to theexternal circuit 117. The external circuit 117 may include a powerdistribution element 164 for regulating power transfer between the fuelcell assembly 102 and various other components of the external circuit117. In particular, the power distribution element 164 may include aplurality of switches configured to selectively connect the fuel cellassembly 102, a power conditioner 166 for a load application 170, abattery 168, an internal load 169, various other suitable components, orsome combination thereof, based on instructions from the processor 162and/or corresponding manual switches. In one example, the internal load169 may be an electrical resistive load.

The power distribution element 164 may be operable based on instructionsfrom a processor 162. During operation of the fuel cell system 100, apower demand of the load application 170 may be communicated to theprocessor. As such, the power distribution element 164 may be configuredto deliver power from the fuel cell assembly 102 to the load application170 via the power conditioner 166. Specifically, the power distributionelement 164 may activate a switch to electrically communicate the fuelcell assembly 102 to the power conditioner 166 and the load application170 so as to deliver power to the load application 170. The powerconditioner 166 may convert the direct current from the fuel cellassembly 102 to an alternating current for supply to the loadapplication 170 (e.g. a power grid, etc.) based on a power demand of theload application 170. Alternately or in addition, the battery 168 may beoperable to supply power to the load application 170. For example,during start-up of the fuel cell system 100, the battery 168 may supplyat least a portion of the power to the load application 170 until thefuel cell assembly 102 commences generating power commensurate with thepower demand of the load application 170.

The fuel cell system 100 may be operated in various modes. A first modemay include delivering power from a fuel cell assembly to the loadapplication 170 commensurate with a power demand of the load application170. Such operation may be referred to as “the power delivery mode.” Inthis mode, the reactant delivery controller 140 may operate the reactantdelivery system 138 to deliver fuel and oxidant to the fuel cellassembly 102 commensurate with a power demand from the external circuit117.

A second mode may include a standby mode in which the fuel cell system100 does not deliver power, but in which the fuel cell stack is kept ina state such that it would be able to go on load and deliver 100% powerin a short period of time.

A third mode may include a shut-down mode in which the fuel cell system100 is completely shut-down, and the fuel supply, temperaturemanagement, etc., are disabled.

It may be appreciated that the processor 162 may include instructionsexecutable to operate the reactant delivery controller 140 and the heatcontroller 147 as well as the power distribution element 164 inaccordance with an operation of the fuel cell assembly 102 in thevarious modes. In one example, the processor 162 may constitute acontroller for controlling fuel cell operation. Operation of the fuelcell system 100 in some of these modes may result in degradations in thefuel cell 103. In one particular embodiment, performance of the fuelcell 103 may decay during operation in a standby mode as a result ofdegradations in a PBI membrane included in the electrolyte 104, asdemonstrated below.

Referring now to FIG. 2, additional detail of an exemplary fuel cellincluding various fuel cell components and materials that may be used asfuel cell 103 is illustrated. In the embodiment described herein, theelectrolyte 104 of the fuel cell 103 includes a PBI membrane, asdescribed above. In addition, FIG. 2 schematically illustrates anexemplary degradation process of the PBI membrane in the fuel cell 103as a result of operation in a standby mode. FIG. 2A shows the fuel cell103 prior to degradation. FIG. 2B shows the fuel cell 103 after the PBImembrane exhibits some degradation, and FIG. 2C shows the fuel cell 103after the PBI membrane exhibits severe degradation. The degradationprocess described below schematically illustrates a typical degradationprocess and described the conditions present during a standby mode thatmay facilitate such a degradation process for a PBI membrane fuel cell.

Turning first to FIG. 2A, the fuel cell 103 as shown prior todegradation includes a PBI membrane 201 as the electrolyte 104 disposedbetween the cathode electrode 106 and the anode electrode 108. Inaddition, the cathode electrode 106 may include a gas diffusion layerconfigured to distribute oxidants, such as air, directed to the cathodeelectrode 106 via flow channels 202 of the cathode flow field plate 110and to transport electrons between ionic species in the electrolyte 104and contact posts 204 of the cathode flow field plate 110. Similarly,the anode electrode 108 may also include a gas diffusion layerconfigured to distribute fuel 108 and transport electrons.

In one embodiment, the fuel cell system 100 including the fuel cell 103described above may be used in back-up power applications where the fuelcell operates between approximately 160 to 180 degrees Celsius inresponse to the power system line voltage and/or current falling belowdesired levels. For back-up power applications where the system mayremain primarily in a standby mode, the stacks may be maintained warm ornear its operating temperature (e.g. at approximately 120 degreesCelsius or higher) to facilitate on load demands of full power deliverywithin short periods of time. However, maintaining the fuel cellassembly 102 in a standby mode for an extended period of time may resultin membrane degradation of the fuel cell system 100.

The inventors have recognized that performance decay of the fuel cellscan result from deteriorations in the PBI membrane including reducedmechanical integrity of the PBI membrane 201 under these conditionspresent when operating the fuel cell system 100 in a standby mode. ThePBI membrane 201 shown in FIG. 2A schematically illustrates an exemplaryembodiment of the fuel cell 300 including the PBI membrane 201 prior todegradation as a result of operation in a standby mode. In contrast,FIG. 2B and FIG. 2C illustrate exemplary embodiments of the PBI membraneat a degraded state and a fully degraded state with reduced mechanicalintegrity following extended stay in standby mode.

As such, a method for reducing such degradations in the PBI membrane isprovided to reduce or avoid conditions during standby mode that causereduced mechanical integrity of the PBI membrane.

FIG. 2B illustrates the fuel cell after operation in standby mode. Thefuel cell 103 of FIG. 2A may be degraded as indicated in FIG. 2B bydegraded fuel cell 103′. In particular, degraded fuel cell 103′ includesPBI membrane 201 with reduced mechanical integrity as indicated bydegraded PBI membrane 201′ as a result of operation in standby mode.

In one aspect of degradation of the PBI membrane, reduced mechanicalintegrity may result from PBI instability under a PBI ‘cleaving’environment. Specifically, the fuel cell at higher temperature andhigher potential may cleave PBI polymer chains to reduce its molecularweight causing the PBI-PA membrane to change from a form of gel to aliquid solution. Such conditions may be present during non-powergeneration or standby mode because the fuel cell potential increaseswhen power delivery from the fuel cell assembly 102 is discontinued.During power delivery operation, current flow from the fuel cell triesto increase the anode potential and decreases the cathode potentialresulting in reduced fuel cell voltage. In contrast, under non-operatingmode the fuel cell can achieve higher voltages known as Open CircuitVoltage (OCV), which may cause the PBI membrane to become liquefied. Theliquefied membrane may then soak into the gas diffusion layers and/orthe flow channels of the flow field plates. As a result, the thicknessof the degraded PBI membrane 201′may be substantially thinner, as shownin FIG. 2B, after extended stay in the standby mode resulting inperformance decay from increased gas permeability, as one example.

In another aspect of degradation, the mechanical integrity of the PBImembrane may result from the evaporation of water from the phosphoricacid from the PBI membrane. In particular, the phosphoric aciddehydrates at higher temperatures causing an increase of theconcentration of the phosphoric acid in the PBI membrane and formpyrophosphoric and polyphosphoric acid. The solubility of PBI increasesas the concentration of phosphoric acid increases and further increasesunder higher temperatures. As a result of the reduced mechanicalintegrity, the PBI membrane may be pressed into the gas diffusion layersin the anode and cathode electrodes under compression of the fuel cell,again resulting in thinning of the PBI membrane

The above degradations related to the PBI membrane structure may resultin excessive thinning, causing permanent degradation of the fuel cellsystem. Specifically, thinning of the PBI membrane may occur to a degreecausing the fuel cell system to be substantially inoperable. FIG. 2Cillustrates the fuel cell after excessive thinning resulting in“shorting” of the anode and the cathode electrodes. The fuel cell 103 ofFIG. 2B may be fully degraded as indicated by fully degraded fuel cell103″ and, as such, may include the PBI membrane in a fully degradedstate as indicated by fully degraded PBI membrane 201″.

The term “shorting” of the fuel cell may hereinafter refer todegradation of the electrolyte between the anode and the cathodeelectrodes resulting in physically touching of the anode and cathodeelectrode causing an unintended internal path for electrical currentbetween these components.

Permanent degradation of the system may result from shorting of the fuelcell through a fully degraded PBI membrane 201″. Fully degraded fuelcell 201″ may not hold a voltage for power delivery. Additional dataillustrating the catastrophic effects of such degradation are providedbelow herein with regard to FIG. 4. Further, such degradation may alsobe affected by standby temperature, also illustrated below herein withregard to FIG. 5.

Referring now to FIG. 3, a high level flowchart is shown that describesoperation of the fuel cell 103 by the processor 162 during a transitionout of power delivery or generation mode into a standby mode, where acombination of coordinated temperature management, cell voltagemanagement, and reactant supply management is used. The approach mayavoid use of blanket of inert gasses, thus avoiding secondary gas supplyand storage. In one specific example, a process is used where, duringoperation in the standby mode, oxidants are first depleted from the fuelcell cathode electrode by consuming the oxidant via an electrochemicalprocess. Then, additional fuel may be supplied to the fuel cell tocompensate for any back-diffusion of oxygen into the fuel cell. In thisway, the above described degradation mechanisms of the PBI membrane maybe avoided or reduced during extended exposure in the standby mode.

Referring now specifically to FIG. 3, method 300 is initiated at 301when the processor162 first detects a shutdown command. The processormay receive a shutdown signal in response to various situations. Undersome conditions, operation of the fuel cell system in the power deliverymode may be at least temporarily interrupted. For example, the powerdemand from the load application may cease or when another sourcecommences delivery of power to the load application. In another example,where the fuel cell system functions as a back-up power source, powerrequired from the fuel cell assembly may cease or when a primary powersource (such as an electrical grid) becomes operable following a failurecausing the shutdown of the fuel cell system. In yet another example,the fuel cell assembly may be disconnected from the load application dueto an emergency shutdown of the fuel cell system based, for example, ona maintenance recommendation triggered by a detected condition of thefuel cell system. Such conditions include without limitation an abnormalfuel flow, temperature condition, irregular voltage, etc.

Following a detected shutdown signal at 301, the external load isremoved from the fuel cell assembly. The power distribution element mayconfigure a switch to electrically disconnect the fuel cell assemblyfrom the load application.

At this time, performance decay of the fuel cells may result fromdeteriorations in the PBI membrane as a result of the PBI instabilityand increased concentration of phosphoric acid under the highertemperature conditions and electrochemical potential present duringstandby. To mitigate these conditions and reduce thinning of the PBImembrane, method 300 includes at 304 measures for creating safeconditions for the PBI membrane as described below.

Continuing with FIG. 3, upon removing the external load, method 300 mayinclude a process 304 for creating safe conditions for the PBI membrane.Specifically, process 304 may include:

-   -   A: First prevent exposure to high electrochemical potential.        Reduction of the cell potential may be achieved in a number of        ways. Higher potentials are generally present while oxidants        remain within the fuel cell. Following removal of the external        load, unconsumed oxidant from previous power generation mode may        remain in the fuel cell. As such, one way to reduce the cell        potential includes consuming reactants, such as oxidants,        remaining in the fuel cell from previous power generation        operation. Various electrochemical processes may be used to        consume the oxidant, such as applying an internal load to the        fuel cell (e.g., between the anode and cathode) to facilitate a        reaction to consume the residual oxidant. Hydrogen or any other        species that react with oxygen may also be supplied to the        cathode electrode to consume the remaining oxidant. The other        alternative may include use of an inert gas, like nitrogen, to        displace residual oxygen from the fuel cell.

Method 300 may also include:

-   -   B: reducing temperature of the fuel cell system. Specifically,        the heat control system may operate to prompt the fan 152 to        cool the fuel cell assembly to a predetermined temperature as        detected by the fuel cell temperature sensor 158.

In this way, the fuel cell system enters a standby mode, as shown at306, with safe conditions for the PBI membrane. However, such safeconditions may be compromised if oxidant back-diffuses to the cathodeand/or anode of the fuel cell resulting in increases in cell potential.Nevertheless, safe conditions may be maintained by monitoring the fuelcell for higher potentials and desired temperatures at 308 and applyingthe processes at 304 when the potential or temperature of the fuel cellincreases outside of safe conditions for the PBI membrane. Specifically,if it is determined at 308 that the cell potential or temperature fallsoutside of safe conditions at, then the answer at 308 is yes, and method300 returns to 304, and the system performs the processes describedabove. If no, method 300 continues to monitor for higher cell potentialsand temperatures at 308.

In some embodiments, method 300 may determine higher potentials upondetecting voltages above a predetermined voltage. If the voltage of thefuel cell increases above a predetermined voltage, such as 0.4 V, theanswer at 308 is yes, and method 300 returns to 304. As describedherein, the voltage of the fuel cell may be detected and used to promptthe process for reducing potential. However, the process described at304 may be prompted based on various other conditions, such as currentthrough the internal load, pressure, a duration since the load removal,etc., that indicate cell potentials outside of safe conditions. In oneembodiment, the method may comprise periodically adding a predeterminedamount of hydrogen at predetermined time intervals where thepredetermined time intervals may be based on experimentation and/or themodels associated with an oxidant-consuming reaction. The fuel may beadded continuously, intermittently, or in any other suitable manner. Inone particular example, the method may trickle hydrogen flow into theanode to sustain a small current flow through the internal load. Inthese ways, the internal load may be used to equalize the cell, andfurther maintain the cell equilibrium while supplying the hydrogen usedto compensate for the back-diffused oxidant.

Temperature control may include heat exchange to and from the fuel cellassembly to maintain safe temperature conditions within a desiredtemperature range. The temperature range may allow the PBI membrane toremain warm enough to rapidly enter power delivery mode from standbymode during a subsequent start-up of the fuel cell system while stillmaintaining safe conditions for the PBI membrane. In one embodiment, thetemperature may be maintained, between 50-120 degrees C. During suchtemperature control, if the fuel cell is heated to a temperature above apredetermined temperature, the answer at 308 is yes, and method 300returns top 304. At this time, temperature control may include promptingthe fan 152 to cool the fuel cell assembly if the fuel cell temperaturesensor 158 detects a temperature higher than a maximum temperature at308 and turning the fan 152 off when a minimum temperature is detected.

Referring now to FIG. 4, graph 400 demonstrates performance decay of afuel cell including a PBI membrane during standby mode following ashutdown, without the compensation of FIG. 3. In particular, graph 400shows a decay rate of the voltage of the fuel cell, as demonstrated ataxis 402, at various standby temperatures, as shown at axis 404, for agiven time in standby mode. As such, data for the fuel cell shown ingraph 400 may include performance decay attributable to membranedegradations as described above. It may be understood that propheticdata 406 may be extrapolated from experimental data 408.

Referring now to FIG. 5, graph 500 demonstrates decay of an open circuitvoltage for a fuel cell including a PBI membrane at varioustemperatures. In particular, graph 500 shows data of open circuitvoltages of a fuel cell including the PBI membrane over time indicativeof degradations and a gross failure of the PBI membrane.

In general, graph 500 demonstrates that increased decay of open circuitvoltages (OCV) in a fuel cell including the PBI membrane may occurwithin reduced intervals of time when the fuel cell remains at opencircuit voltage at increased temperatures. Graph 500 includes an axisfor demonstrating a voltage of the cell at 502 and an axis fordemonstrating a length of time that the fuel cell remains in standbymode during uncontrolled shutdown at 504. Specifically, graph 500 showsthe open circuit voltage of the fuel cell over time at 80 degreesCelsius at data curve 506, at 100 degrees Celsius at data curve 508, at120 degrees Celsius at data curve 510, and at 160 degrees Celsius atdata curve 512.

Data curves 506, 508, and 510 may demonstrate increased rates of decayof the fuel cell open circuit voltage due to PBI membrane degradation.Specifically, a rapid drop in open circuit voltage as a result offailure in the mechanical integrity of the PBI membrane is notdemonstrated in the data of curves 506, 508, and 510. For example, datacurve 506 shows that open circuit voltages with relatively reduced decaymay be demonstrated when the temperature of the fuel cell isapproximately 80 degrees Celsius. Data curve 508 shows that open circuitvoltages with slightly increased rates of decay relative to data curve506 may be demonstrated when the temperature of the fuel cell isapproximately 100 degrees Celsius; however, even at a temperatureapproximately corresponding to the boiling point of water, the opencircuit voltage may still demonstrate relatively reduced decay. Datacurve 510 may show further increased decay of open circuit voltages;however, a voltage may still be sustained during the interval of timeshown.

Data curve 512 demonstrates gross failure of the PBI membrane resultingfrom one or more of the above aspects of degradation and asschematically illustrated in fully degraded PBI membrane 201″ of FIG.2C. As shown by the exemplary data curve at 512, gross failure of thePBI membrane may be understood to be associated with a rapid drop of anopen circuit voltage. In particular, gross failure of the PBI membranemay generally be responsive to mechanical failure of the PBI membranerather than various other degradations in the fuel cell. Such a rapiddrop may result from electrical communication between the anode and thecathode as a result of excessive thinning of the PBI membrane. The rapidthinning of the PBI membrane may also result in substantial mixing ofthe reactants at the anode and cathode, such as through a void in theelectrolyte, such that depolarization of the fuel cell may facilitate arapid drop in voltage from the time of gross failure at dashed line 514to the loss of voltage at dashed line 516. As such, degradation of thefuel cell described herein may experience such a failure as a result ofreduced mechanical integrity of the membrane that may apply uniquely toan electrolyte including a PBI membrane.

Although the present disclosure includes specific embodiments, specificembodiments are not to be considered in a limiting sense, becausenumerous variations are possible. The subject matter of the presentdisclosure includes all novel and non-obvious combinations andsubcombinations of the various elements, features, functions, and/orproperties disclosed herein. The following claims particularly point outcertain combinations and subcombinations regarded as novel andnon-obvious. These claims may refer to “an” element or “a first” elementor the equivalent thereof. Such claims should be understood to includeincorporation of one or more such elements, neither requiring norexcluding two or more such elements. Other combinations andsubcombinations of features, functions, elements, and/or properties maybe claimed through amendment of the present claims or throughpresentation of new claims in this or a related application. Suchclaims, whether broader, narrower, equal, or different in scope to theoriginal claims, also are regarded as included within the subject matterof the present disclosure.

1. A method for reducing degradation in a fuel cell assembly including apolybenzimidazole (PBI) membrane doped with phosphoric acid, the methodcomprising: electrochemically consuming oxidant from a cathode coupledto the PBI membrane in response to a disconnection of an external load;and supplying fuel to maintain zero or near zero potential by removingany back-diffused oxidant to the associated anode or cathode electrodeof the fuel cell sufficient to consume or replace the back-diffusedoxidant while the external load is disconnected.
 2. The method of claim1 where the fuel is supplied without a supply of inert gas to the anodeand/or cathode, and where the fuel is supplied at a predetermined rate.3. The method of claim 2 further comprising reducing the fuel celltemperature to a predetermined temperature while maintaining low cellpotential, where the predetermined temperature is lower than anoperating temperature of the fuel cell.
 4. The method of claim 1 furthercomprising discontinuing an oxidant supply in response to disconnectionof the external load.
 5. The method of claim 1 where theelectrochemically consumed oxidant includes oxidant supplied duringprevious operation of the fuel cell in power delivery mode.
 6. Themethod of claim 2 where an internal load is applied to the fuel cell toelectrochemically consume the oxidant.
 7. The method of claim 6 wherethe internal load is further applied during the removal of theback-diffused oxidant.
 8. The method of claim 1 where the removal of theexternal load is determined when a detected voltage of the fuel cell isgreater than or equal to a threshold value.
 9. The method of claim 1further comprising reducing the fuel cell temperature below apredetermined temperature; and maintaining the fuel cell temperature atthe predetermined temperature while the external load is disconnected.10. A fuel cell system, comprising: at least one fuel cell stack, wherea plurality of cells in the stack include a polybenzimidazole (PB I)membrane electrolyte having an anode and a cathode; flow fieldsconfigured to supply air to the cathode and hydrogen to the anode of thefuel cell system; an external load; an internal load; a controllerconfigured to detect disconnection of the external load and transitionthe system from power generation to a stand-by mode, stop a supply ofoxidant, connect the internal load in response to the detected externalload disconnection, the internal load reducing voltage of the fuel cellto substantially zero, maintain connection of the internal load whilesupplying hydrogen to the anode of the fuel cell, where the hydrogen isrepeatedly supplied commensurate with a reaction of back-diffused oxygenat the cathode, and reducing a temperature of the fuel cell to apredetermined temperature, without supplying inert gasses during saidmaintenance of zero or near zero potential.
 11. The fuel cell system ofclaim 10 where the system is coupled in a back-up power generationsystem, where the controller operates the fuel cell to provide back-uppower in response to line voltage and/or current of a power system. 12.The fuel cell system of claim 10 where the hydrogen is repeatedlysupplied at a predetermined rate.
 13. The fuel cell system of claim 10where the disconnection of the external load is detected when a voltageof the fuel cell is greater than or equal to a threshold value.
 14. Themethod of claim 9, where temperature is reduced by adjusting the coolingfan.
 15. The method of claim 6 further comprising detectingdisconnection of the external load, and transitioning from powergeneration to a standby mode, stopping a supply of oxidant, connectingthe internal load in response to the detected external loaddisconnection, maintaining connection of the internal load whilerepeatedly supplying hydrogen to the anode of the fuel cell, andsupplying the hydrogen commensurate with the reaction of back-diffusedoxygen at the cathode, without supplying inert gasses during maintenanceof zero or near zero potential.
 16. The method of claim 15 furthercomprising detecting the disconnection of the external load when avoltage of the fuel cell is greater than or equal to a threshold value.